Real and Imaginary Properties of Epsilon-near-Zero Materials
Mohammad H. Javani and Mark I. Stockman
Department of Physics and Astronomy and Center for Nano-Optics, Georgia State University, Atlanta, GA 30340, USA
E-mail address:mstockman@gsu.edu
Abstract: From the fundamental principle of causality we show that epsilon-near-zero (ENZ) materials with very low (asymptotically zero) intrinsic dielectric loss do necessarily possess a very low (asymptotically zero) group velocity of electromagnetic wave propagation. This leads to the loss function being singular and causes high non-radiative damping of optical resonators and emitters (plasmonic nanoparticles, quantum dots, chromophore molecules) embedded into them or placed at their surfaces. Rough ENZ surfaces do not exhibit hot spots of local fields suggesting that surface modes are overdamped. Reflectors and waveguides also show very large losses both for realistic and idealized ENZ.
1. Introduction
Recently, materials at frequencies close to the bulk plasmon frequency, , which are characterized by dielectric permittivity being small enough, , and are usually referred to as Epsilon Near Zero (ENZ) materials, have attracted a great deal of attention, see, e.g., [1-3].
Their optical properties are expected to be quite remarkable: ENZ should totally reflect light at all angles, the phase velocity of light tends to infinity and, correspondingly, the light wave carries almost constant phase, the density of photonic states diverges at , a waveguide formed inside an ENZ material can confine light at deep sub-wavelength dimensions, there is no reflections even at sharp bands, and the unavoidable roughness of the waveguide walls does not significantly spoil the wave-guiding. As in many other cases in nanooptics \cite[4], dielectric losses present a significant problem deteriorating these unique properties and limiting useful applications of ENZ materials.
2. Results
Following Ref. [5], we show that the fundamental principle of causality [as given by dictates that any ENZ material with a very low (asymptotically zero) loss at the observation frequency has necessarily asymptotically zero group velocity at that frequency. Physically, this leads to enhanced scattering and dissipative losses as given by the diverging energy-loss function. Paradoxically, a reduction of the intrinsic loss, , leads to an increase of energy-loss function and further deterioration of performance of reflectors and waveguides built from ENZ materials. Both analytically and numerically we have shown that a realistic ENZ material ITO at the bulk plasma frequency causes high reflection and propagation losses. The singular loss function is also responsible for anomalously strong optical damping of resonant systems (plasmonic nanoparticles, dye molecules, quantum dots, etc.) embedded into or positioned at the surfaces of ENZ materials. In contrast to plasmonic metals, there are no pronounced hot spots of local fields at rough ENZ surfaces. Structured dielectric media with practically zero loss in the optical region cannot function as true ENZ materials because of the singular response; they necessarily are diffractive photonic crystals, and not refractive effective media. Obviously, this anomalous loss of ENZ materials can be gainfully used in energy absorbers, which begets analogy with heating of plasmas at plasma frequency with charged particles or electromagnetic waves. These losses and singularities are fundamental, local properties of the ENZ media, which cannot be eliminated by micro- or nano-structuring.
4. References
1. N. Engheta, Pursuing near-Zero Response, Science 340, 286-287 (2013).
2. R. Marques, J. Martel, F. Mesa, and F. Medina, Left-Handed-Media Simulation and Transmission of Em Waves in Subwavelength Split-Ring-Resonator-Loaded Metallic Waveguides, Phys. Rev. Lett. 89, 183901-1-4 (2002).
3. M. Silveirinha and N. Engheta, Tunneling of Electromagnetic Energy through Subwavelength Channels and Bends Using Epsilon-near-Zero Materials, Phys. Rev. Lett. 97, 157403 (2006).
4. M. I. Stockman, Nanoplasmonics: Past, Present, and Glimpse into Future, Opt. Express 19, 22029-22106 (2011).
5. M. H. Javani and M. I. Stockman, Real and Imaginary Properties of Epsilon-near-Zero Materials, Phys. Rev. Lett. 117, 107404-1-6 (2016).

We show the existence of an inherent property of evanescent electromagnetic waves: spin-momentum locking, where the direction of momentum fundamentally locks the polarization of the wave. We trace the ultimate origin of this phenomenon to complex dispersion and causality requirements on evanescent waves. We demonstrate that every case of evanescent waves in total internal reflection, surface states and optical fibers/waveguides possesses this intrinsic spin-momentum locking. We also introduce a universal right-handed triplet consisting of momentum, decay and spin for evanescent waves. We derive the Stokes parameters for evanescent waves which reveal an intriguing result - every fast decaying evanescent wave is inherently circularly polarized with its handedness tied to the direction of propagation. We also show the existence of a fundamental angle associated with total internal reflection (TIR) such that propagating waves locally inherit perfect circular polarized characteristics from the evanescent wave. This circular TIR condition occurs if and only if the ratio of permittivities of the two dielectric media exceeds the golden ratio. Our work leads to a unified understanding of this spin-momentum locking in various nanophotonic experiments and sheds light on the electromagnetic analogy with the quantum spin hall state for electrons.

The optical response of noble-metal nanoparticles in the visible spectrum is characterized by the presence localized surface plasmon resonances. Localized surface plasmons are non-propagating coherent oscillations of free-carriers coupled to the electromagnetic field arising as a consequence of confinement effects in sub-wavelength nanoparticles. Plasmonic nanoparticles in general support an infinite discrete set of orthogonal localized surface plasmon modes, yet in the case of structures of deep-subwavelength dimensions only the lowest order resonances of dipolar nature can be effectively excited by an incident electromagnetic wave. By reciprocity such high-order modes tend to be subradiant and therefore difficult to observe in far-field. Here we discuss the novel localized surface plasmon dynamics that emerge when the electromagnetic properties of the plasmonic particle or of the background medium vary in time. We show in particular that such temporal permittivity variations lift the orthogonality of the localized surface plasmon modes and introduce coupling among different angular momentum states. Exploiting such dynamics we show how surface plasmon amplification of high order resonances can be achieved under the action of a spatially uniform optical pump of appropriate frequency.

High-power femtosecond filaments—laser-light beams capable of kilometer-long propagation—attract interest of nonlinear-optics community due to their numerous applications in remote sensing, lightning protection, virtual antennas, and waveguiding. Specific arrangements of filaments, into waveguides or hyperbolic metamaterials, allow for efficient control and guiding of electromagnetic radiation, radar-beam manipulation, and resolution enhancement. These applications require spatially uniform distribution of densely packed filaments.
In order to address this challenge, we investigate the dynamic properties of large rectangular filament arrays propagating in air depending on four parameters: the phase difference between the neighboring beams, the size of the array, separation between the beams, and excitation power. We demonstrate that, as a result of the mutual interaction between the filaments, the arrays where the nearest neighbor beams are out-of-phase are more robust than the arrays with all the beams in phase.
Our analysis of the array stability reveals that there exist certain trade-offs between the stability of a single filament and the stability of the entire array. We show that in the design of the experiment, the input parameters have to be chosen in such a way that they ensure a sufficiently high filling fraction, but caution has to be used in order not to compromise the overall array stability.
In addition, we show the possibility of filament formation by combining multiple beams with energies below the filamentation threshold. This approach offers additional control over filament formation and allows one to avoid the surface damage of external optics used for filamentation.

A new methodology is proposed to implement laser beam steering with a wide angle of view and 4-5 orders of magnitude enhancement in scanning speed. It is based on light-matter interaction between metasurfaces and mode-locked lasers with a frequency-comb spectrum (i.e., equally-spaced phased-locked frequency lines). Replacing CW lasers with frequency-comb sources expands the impact of flat metasurfaces towards producing dynamic optical patterns rather than only static patterns in the far-field. A metasurface is judiciously designed to produce the optical pattern of a rotating light beam realizing ultrafast laser scanning with ~90-deg angle-of-view over ~100ps time interval.

Measurements are presented which examine the use of gaseous plasma elements as highly-tunable resonators. The resonator considered here is a laser-induced plasma kernel generated by focusing the fundamental output from a Q-switched Nd:YAG laser through a lens and into a gas at constant pressure. The near-ellipsoidal plasma element interacts with incoming microwave radiation through excitation of low-order, electric-dipole resonances similar to those seen in metallic spheres. The tunability of these elements stems from the dispersive nature of plasmas arising from their variable electron density, electron momentum transfer collision frequency, and the
concomitant e↵ect of these properties on the excited surface plasmon resonance. Experiments were carried out in the Ku band of the microwave spectrum to characterize the scattering properties of these resonators for di↵erent values of electron density. The experimental results are compared with results from theoretical approximations
and finite element method electromagnetic simulations. The described tunable resonators have the potential to be used as the building blocks in a new class of all-plasma metamaterials with fully three-dimensional structural flexibility.

The ability to engineer the optical phase at subwavelength dimensions has led to metasurfaces that provide unprecedented control of electromagnetic waves. To reach their ultimate potential, metasurfaces must incorporate reconfigurable functions. The central challenge is achieving large tunability in subwavelength elements. Here, we describe two different approaches for achieving order-unity refractive index shifts: free-carrier refraction and thermo-optic tuning. We experimentally demonstrate wide tuning of single-particle infrared Mie resonances through doping, and demonstrate simulations of electrically reconfigurable III-V heterojunction metasurfaces based on these effects. We conclude with recent experimental demonstrations of dyamic, ultrawide tuning of Mie resonators based on two distinct thermo-optic effects: 1) modifying the electron mass and carrier density in InSb and 2) exploiting the anomalous temperature-dependent bandgap of PbTe.

The past few years have witnessed the discovery of photonic topological insulators, which transformed our views on propagation and scattering of electromagnetic waves. Here we show that judiciously designed all-dielectric photonic metamaterials and metasurfaces can exhibit a topological photonic state. The bianisotropy of the metamaterial is shown to result in emergence of a topological photonic band gap in the bulk and photonic states guided by edges and interfaces which appear to be one-way spin-polarized and immune to sharp bending. Experimental realizations of photonic topological metamaterials and metasurfaces for microwave and optical domains are used to test and confirm the topological properties.

The main achievement of the modern plasmonics is the concentration of light into nanospots that are much smaller than the wavelength. Nanospot concentration is beneficial for various applications: biomedical imaging and sensing, optical microscopy with single-molecule resolution, heat assisted magnetic recording (HAMR), QED studies, nanolasing, etc. Until now, plasmonic metal nanoantennae, sub-wavelength apertures or metallic near field concentrators (NFCs) are used for this purpose. The main advantage of the metal NFC is their capabilities to localize plasmonic modes, which can be excited by the incident transverse em wave. However, the metal NFCs have large optic loss so we propose a novel all-dielectric NFC, which allows focusing the light into a sub-wavelength hot nanospot, without the dissipative loss. The detrimental dephasing and thermal effects almost vanishes in the dielectric NFC opening new opportunities in the magnetic recording and quantum plasmonics. The ability to concentrate light is important not only to fundamental physics studies, but also to practical device applications. For example, microcavities can force the atoms or quantum dots to emit spontaneous photons in a desired direction or can provide an environment, where dissipative mechanisms such as spontaneous emission are overcome. The electric field is much enhanced in the proposed new device at the vertex of the dielectric beak, which is attached to the tablet dielectric resonator. The resonator in turn is pumped through the plane waveguide. The electric field is enhanced due to longitudinal polarization of the beak vertex, which is excited by em field of the pumped resonator.

Recently, a novel class of high-Q optical resonators based on all-dielectric subwavelength nanoparticles with high refractive index has been proposed [M. V. Rybin, et al, arXiv:1706.02099, 2017]. Here we study a complex spectrum of such nanoscale resonators by means of the resonant-state expansion, treating the problem as a nonHermitian eigenproblem. We show that the high-Q features can be described within the mechanism of external coupling of open channels via the continuum. For ceramic resonators with permittivity ε = 40, we demonstrate that the quality factor of a trapped mode with a low azimuthal index could reach the value Q = 104 .

The emerging field of metasurfaces is promising to realize novel optical devices with miniaturized flat format and added functionalities. Metasurfaces have been demonstrated to exhibit full control of amplitude, phase and polarization of electromagnetic waves. Using the metasurface, the wavefront of light can be manipulated permitting new functionalities such as focusing and steering of the beams and imaging. One optical component which can be designed using metasurfaces is the axicon. Axicons are conical lenses used to convert Gaussian beams into nondiffraction Bessel beams. These unique devices are utilized in different applications ranging from optical trapping and manipulation, medical imaging, and surgery. In this work, we study axicon lens design comprising of planar metasurfaces which generate non-diffracting Bessel beams at visible wavelengths. Dielectric metasurfaces have been used to achieve high efficiency and low optical loss. We measured the spot size of the resulted beams at different planes to demonstrate the non-diffraction properties of the resulted beams. We also investigated how the spot size is influenced by the axicon aperture. Furthermore, we examined the achromatic properties of the designed axicon. Comparing with the conventional lens, the metasurface axicon lens design enables the creation of flat optical device with wide range of depth of focus along its optical axis.

Dielectric nanoantennas and metasurfaces have proven to be able to manipulate the wavefront of incoming waves with high transmission efficiency. The important next question is: Can they enable enhanced interaction with the light to transform its colour or to be able to control one light beam with another? Here we show how a dielectric nano-resonator of subwavelength size can enable enhanced light matter interaction for efficient nonlinear frequency conversion. In particular, we show how AlGaAs or silicon nanoantennas can enhance second and third harmonic generation, respectively. Importantly, by controlling the size of the antennas, we can achieve control of directionality and polarisation state of the emission of harmonics. Our results open novel applications in ultra-thin light sources, light switches and modulators, ultra-fast displays, and other nonlinear optical metadevices based on low loss subwavelength dielectric resonant nanoparticles.

Nanoparticles composed of high refractive index semiconductors can support electric and magnetic multipolar Mie-type resonances that can be tuned by the nanoresonator design [1]. Furthermore, such semiconductor nanoresonators can exhibit very low absorption losses at optical frequencies. Based on these properties, semiconductor nanoresonators represent versatile building blocks of functional photonic nanostructures with tailored optical response.
This talk will review our recent advances in controlling the generation and propagation of light with metasurfaces composed of high-index semiconductor nanoresonators. Such metasurfaces can impose a spatially variant phase shift onto an incident light field, thereby providing control over its wave front with high transmittance efficiency [2]. However, there are two important limitations: most semiconductor metasurfaces realized so far are passive, and their optical response is permanently encoded into the structure during fabrication. This talk will concentrate on strategies to integrate emitters into the metasurfaces and to obtain dynamic control of the metasurface optical response.
In particular, two approaches for active tuning of the metasurface response will be discussed, namely integration of the metasurface into a nematic-liquid-crystal cell [3] and ultrafast all-optical tuning based on the nonlinear optical response of the constituent semiconductor materials. Furthermore, I will show that Mie-resonant semiconductor metasurfaces allow for spatial and spectral tailoring of spontaneous emission from various types of emitters.
[1] M. Decker & I. Staude, J. Opt. 18, 103001 (2016).
[2] K. E. Chong et al., Nano Lett. 15, 5369–5374 (2015).
[3] J. Sautter et al., ACS Nano 9, 4308–4315 (2015).

Optical nanoantennas possess great potential for controlling the spatial distribution of light in the linear regime as well as for frequency conversion of the incoming light in the nonlinear regime. However, the usually used plasmonic nanostructures are highly restricted by Ohmic losses and heat resistance. Dielectric nanoparticles like silicon and germanium can overcome these constrains [1,2], however second harmonic signal cannot be generated in these materials due to their centrosymmetric nature. GaAs-based III-V semiconductors, with non-centrosymmetric crystallinity, can produce second harmonic generation (SHG) [3]. Unfortunately, generating and studying SHG by AlGaAs nanocrystals in both backward and forward directions is very challenging due to difficulties to fabricate III-V semiconductors on low-refractive index substrate, like glass. Here, for the first time to our knowledge, we designed and fabricated AlGaAs nanoantennas on a glass substrate. This novel design allows the excitation, control and detection of backwards and forwards SHG nonlinear signals. Different complex spatial distribution in the SHG signal, including radial and azimuthal polarization originated from the excitation of electric and magnetic multipoles were observed. We have demonstrated an unprecedented SHG conversion efficiency of 10-4; a breakthrough that can open new opportunities for enhancing the performance of light emission and sensing [4].
References
[1] A. S. Shorokhov et al. Nano Letters 16, 4857 (2016).
[2] G. Grinblat et al. Nano Letters 16, 4635 (2016).
[3] S. Liu et al. Nano Letters 16, 7191 (2016).
[4] R. Camacho et al. Nano Lett. 16, 7191 (2016).

Due to the weak magnetic responsibility of natural existing materials at optical frequency, optical magnetism remains a “dark state” of light which is largely unexplored. However, optical magnetism is also very desirable because of the many splendid possibilities it may lead to, including ultra-compact opto-magnetic storage devices, high speed magnetic imaging, magnetic tweezers etc. Here we design a Si nano-disk structure as the magnetic nanoprobe which supports magnetic resonance in visible range with the incident azimuthally polarized beam (APB). APB features a donut shape beam profile, with a strong longitudinal magnetic field and a vanishing electric field at the beam axis. Therefore, on the magnetic resonance while the probe is aligned to the APB axis, a longitudinal magnetic dipole is excited in the probe, and interacts with the incident APB inducing an exclusive magnetic force. Making such magnetic nanoprobe under APB illumination serves as an important first step to realize the proposed photoinduced magnetic force microscopy (PIMFM), which selectively exploits the interaction between matter and the magnetic field of light to characterize the optical magnetism in nanoscale. Such investigation of the optical magnetism in samples is dearly needed in many mechanical, chemical, and life-science applications.

A semiconductor-microcavity is an optical structure composed of two mirrors separated by a layer of semiconducting material. If the energy of the confined photon and excitonic transition are degenerate, interactions can occur in the strong-coupling regime, with the eigenstates of the system being cavity polaritons (a coherent superposition between light and matter).
Due to their bosonic nature, cavity polaritons are able to undergo condensation to a macroscopically occupied coherent state. Polariton condensates can be optically pumped and then undergo decay by emitting coherent light, very much like an optically pumped laser. Here, we demonstrate evidence of polariton condensation in a microcavity containing a dispersed molecular dye.
Our structures are based on two dielectric-mirrors placed either side of a film of the transparent matrix polymer polystyrene containing the fluorescent molecular dye BODIPY-Br. We first show using CW photoluminescence measurements that weakly-coupled excimer-like states in the BODIPY-Br, together with emission from the (0,1) vibrational transition are responsible for optically pumping polariton states along the lower polariton branch.
We then explore the non-linear emission from control thin-films and cavities using pulsed laser excitation. We obtain strong evidence of non-linear photoluminescence with increasing excitation density, associated with a six-fold linewidth narrowing and a continuous blue-shift attributed to polariton interactions with other polaritons and the exciton reservoir. We believe that there is a large number of molecular dyes that could dispersed into a polymer matrix allowing polariton condensation to be realised at wavelengths spanning the entire visible and near infrared.

We will focus on approaches which make use of light-matter interactions to alter the chemical behavior of a target molecular species. This is done through cavity coupling to a molecular vibration. Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations. This approach is a viable method to explore controlling chemical reactivity and energy relaxation. Here, we demonstrate frequency domain results for vibrational bands strongly coupled to optical cavities. We experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and several molecular species (e.g., poly-methylmethacrylate, thiocyanate, hexamethyl diisocyanate). We investigate strong and weak coupling regimes through examination of cavities loaded with varying concentrations of a urethane monomer. Rabi splittings are in excellent agreement with an analytical description using no fitting parameters. We show that coupling strength is a function of molecule/cavity mode overlap by systematically altering the position of a molecular slab throughout a first order cavity with results agreeing well with analytical and transfer matrix predictions. Further, remote molecule-molecule interaction will be explored by placing discrete and separated molecular layers throughout a cavity. In addition to establishing that coupling to an optical cavity modifies the energy levels accessible to the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.

Reconfigurable Mie resonator metasurfaces may give rise to new classes of programmable optical devices. Large phase and amplitude modulations can be achieved with high-Q resonances that are tunable by at least one line-width. We experimentally demonstrate narrow linewidth, reconfigurable Mie resonators comprising undoped InSb wires embedded inside a highly doped InSb Epsilon-Near-Zero (ENZ) cavity. We demonstrate a Q-factor increase of 400% by embedding a high index resonator within, instead of atop, an ENZ substrate. Systematic studies of varying width resonators reveal significant differences in coupling to the ENZ media for TM and TE resonators. A large refractive index modulation (Δn ≥1.5) is achieved with heating (80-575K), stemming from variations in the effective mass of free-carriers. Thermally tuning the ENZ wavelength of the cavity by >2μm (13-15.5μm) emables reconfigurable tuning by multiple line-widths. This ultra-wide thermal tunability of high-Q embedded resonators may enable new class of active metadevices in the mid-infrared wavelength regime.

The control of light-matter interaction through the use of subwavelength structures known as metamaterials has facilitated
the ability to control electromagnetic radiation in ways not previously achievable. A plethora of passive metamaterials as
well as examples of active or tunable metamaterials have been realized in recent years. However, the development of
tunable metamaterials is still met with challenges due to lack of materials choices. To this end, materials that exhibit a
metal-insulator transition are being explored as the active element for future metamaterials because of their characteristic
abrupt change in electrical conductivity across their phase transition. The fast switching times (&utri;t < 100 fs) and a change
in resistivity of four orders or more make vanadium dioxide (VO2) an ideal candidate for active metamaterials. It is known
that the properties associated with thin film metal-insulator transition materials are strongly dependent on the growth
conditions. For this work, we have studied how growth conditions (such as gas partial pressure) influence the metalinsulator
transition in VO2 thin films made by pulsed laser deposition. In addition, strain engineering during the growth
process has been investigated as a method to tune the metal-insulator transition temperature. Examples of both the optical
and electrical transient dynamics facilitating the metal-insulator transition will be presented together with specific
examples of thin film metamaterial devices.

Many conventional optoelectronic devices consist of thin, stacked films of metals and semiconductors. In this presentation, I will demonstrate how one can improve the performance of such devices by nano-structuring the constituent layers at length scales below the wavelength of light. The resulting metafilms and metasurfaces offer opportunities to dramatically modify the optical transmission, absorption, reflection, and refraction properties of device layers. This is accomplished by encoding the optical response of nanoscale resonant building blocks into the effective properties of the films and surfaces. To illustrate these points, I will show how nanopatterned metal and semiconductor layers may be used to enhance the performance of solar cells, photodetectors, and enable new imaging technologies. I will also demonstrate how the use of active nanoscale building blocks can facilitate the creation of active metafilm devices.

Optically resonant dielectric nanostructures represent a new and rapidly developing research direction in nanophotonics [1]. They show plenty of useful functionalities and can complement or even substitute resonant plasmonic nanoparticles for many potential application directions. The main advantages over conventional plasmonics are low losses, wide range of applicable dielectric materials and strong magnetic resonant response. In particular, the last feature opens a broad range of opportunities to control light scattering, transmission, reflection and phase characteristics through designed interference between electric and magnetic resonant modes. This has already led to demonstrations of low-loss dielectric Huygens’ metasurfaces operating with very high efficiencies in transmission mode and generalized Brewster effect showing unconventional behaviour of dielectric metasurface in reflection mode [1]. In this presentation, we will review recent magnetic resonant phenomena obtained with high-index dielectric nanoantennas and metasurfaces and show how this might lead to new functionalities, which cannot be achieved neither with conventional metasurface approaches nor with conventional bulk optics. In particular, we demonstrate how the resonance interference effect can be used to control energy distribution between diffraction orders in a nanoantenna array, which leads to light bending at very high angles of >82 degrees with efficiency >50%. This property is used to design and experimentally demonstrate flat lenses having a free-space numerical aperture (NA) of >0.99, which strongly exceeds NA of existing flat lenses and bulk optics analogues. Applications of these new, ultra-high NA, flat dielectric lenses will also be discussed.
References:
1) A. I. Kuznetsov et al., “Optically resonant dielectric nanostructures”, Science 354, aag2472 (2016).

Lightweight, portable solar blankets, constructed from thin film photovoltaics, are of great interest to
hikers, the military, first responders, and third-world countries lacking infrastructure for transporting
heavy, brittle solar cells. These solar blankets, as large as two square meters in area, come close to
satisfying specifications for commercial and military use, but they still have limited absorption due to
insufficient material efficiency, and therefore are large and too heavy in many cases.
Metasurfaces, consisting of monolayers of periodic and semi-random plasmonic particles patterned in
a scalable manner, are explored to enhance scattering into thin photovoltaic films (currently of
significant commercial and military value), in order to enhance absorption and efficiency of solar
blankets. Without nano-enhancement, absorption is limited by the thickness of the thin photovoltaic
active layer in the long-wavelength region. In this study, lithographically patterned, periodic Al
nanostructure arrays demonstrate experimentally a large absorption enhancement, resulting in a
predicted increase in short-circuit current density of at least 35% and as much as 70% for optimized
arrays atop 200-nm amorphous silicon thin films. Optimized arrays extend thin-film absorption to the
near infrared region. This impressive absorption enhancement and predicted increase in short-circuit
current density may significantly increase the efficiency and reduce the weight of solar blankets,
enabling their use for commercial and military applications.

Metasurfaces constructed from metallic nanostructures can be designed to operate efficiently as coupling structures for
incident optical beams to surface plasmon polaritons (SPPs) propagating thereon. On a semiconductor, metallic
metasurfaces can act simultaneously as a device electrode while ensuring strong optical field overlap with the active
region. Additionally, SPP fields thereon can be confined to sub-wavelength dimensions and significantly enhanced
relative to the exciting field. These features are very attractive for nanoscale optoelectronic device applications, such as
photodetectors and modulators. We discuss recent progress on optoelectronic metasurfaces, particularly recent device
demonstrations for high-speed reflection modulators based on a metal-oxide-semiconductor capacitor structure
exploiting the carrier refraction effect in Si, and for Schottky contact photodetectors on III-Vs and Si.

Dielectric Mie resonators and quantum-confined semiconductors enable an unrivaled control over light absorption and excited electrons. Here, we embed photoluminescent silicon nanocrystals into a planar array of SiO2 nanocylinders, and experimentally demonstrate a powerful concept: the resulting metamaterial preserves the nanocrystal radiative properties and inherits the spectrally-selective absorption properties of the nanocylinders. This hierarchical approach provides increased photoluminescence intensity without plasmonic components. This spectral selectivity of absorption paves the way for an effective light down-conversion scheme to increase the efficiency of solar cells. The demonstrated principle is general and can be applied to other semiconductor quantum dots, ions or molecules.

Directional light flow is fundamental to the development of photonic information processors. One all optical way of generating such nonreciprocal transport involves exploiting the nonlinear Kerr effect within an asymmetric arrangement of high Q resonators. However, current demonstrations involve optical paths that are tens to hundreds of microns in length.
Here, we show that Kerr based nonreciprocal devices can be further miniaturized to the nanoscale by working with Silicon nanoantenna-based metasurfaces. In the subwavelength regime structural asymmetry alone isn’t enough to generate directionally-dependent field amplification. We overcome this limitation by overlapping a sub-radiant electric dipolar mode with a perpendicular super-radiant magnetic dipole. In this case, breaking out-of-plane inversion symmetry leads to nearfield coupling between the two excitations. Because of interference between nearfield and far field magneto-electric coupling, the electric dipole is suppressed (enhanced) for a normally incident plane wave propagating in the backward (forward) direction. When the metasurface is illuminated with powers of a few 100kW/cm2, the electric field strength within the Si becomes sufficient to change its refractive index, red-shifting the narrow transmission dip. For forward excitation the resonance is shifted by a significant portion of the FWHM, making the metasurface transparent. For backward excitation the much smaller shift renders the transmission very low.
We show, for the first time, that bianisotropy provides a means to achieve optical nonreciprocity at the nanoscale. Relying simply on collocated dipolar excitations, our scheme has, in principle, no lower size limitation and could be miniaturised further by making use of gain assisted plasmonics.

Recently introduced generalized Snell’s law provides wide possibilities for wavefront manipulations using metasurfaces. In contrast to conventional blazed gratings, their metasurface-based counterparts are planar (no grooves) and do not require complicated fabrication techniques. However, all previously demonstrated metasurfaces for anomalous reflection have revealed their deficiency due to parasitic energy coupling into non-desired diffraction modes. This negative effect becomes especially pronounced for metasurfaces designed to have a large separation angle between the incident and reflected beams. The reason is the used inaccurate approach for metasurface synthesis. It approximates that the surface is uniform on the sub-wavelength scale and the phase of the local reflection coefficient follows the linear profile dictated by the generalized reflection law. While the former assumption could be made, the latter one, as we show in the presentation, is incorrect. Moreover, the conventional synthesis approach does not take into account requirements on the amplitude of the local reflection coefficient. In the presentation, we will demonstrate an original alternative design scheme for metasurface gratings based on the generalized surface impedance model. It appears that perfect coupling of an incident plane wave into a single reflected plane wave requires energy channeling along the metasurface plane. To verify our design approach, we fabricate and measure an optical metasurface that reflects a normally incident beam at a very steep angle of 80 deg. The comparison of the two approaches shows that our new scheme provides double increase in the efficiency and complete absence of parasitic reflections.

We discuss the possibility of confining electromagnetic energy and enhancing the interactions of light and matter in nanostructures, based on the concept of embedded eigenstates within the radiation continuum. We discuss how metasurfaces and metamaterials may be able to trap light in plain sight, and how lossless structures may be able to store energy in the transient by engaging complex zeros in the scattering response of the system. We also shed light on the role that reciprocity plays in the response of these systems, and how these functionalities may play a pivotal role in low-energy nanophotonic opto-electronic and bio-sensing devices.

Weyl fermions have not been found in nature as elementary particles, but they emerge as nodal points in the band structure of electronic and classical wave crystals. Novel phenomena such as Fermi arcs and chiral anomaly have fueled the interest in these topological points which are frequently perceived as monopoles in momentum space. We demonstrate that generalized Weyl points can exist in a parameter space and we report the first observation of such nodal points in one-dimensional photonic crystals in the optical range. The reflection phase inside the band gap of a truncated photonic crystal exhibits vortexes in the parameter space where the Weyl points are defined and they share the same topological charges as the Weyl points. These vortexes also guarantee the existence of interface states, the trajectory of which can be understood as “Fermi arcs” emerging from the Weyl nodes.

For vortex fiber multiplexing to reach practical commercial viability, simple silicon photonic interfaces with vortex fiber will be required. These interfaces must support multiplexing. Toward this goal, an efficient singlefed multimode Forked Grating Coupler (FGC) for coupling two different optical vortex OAM charges to or from the TE0 and TE1 rectangular waveguide modes has been developed. A simple, apodized device implemented with e-beam lithography and a conventional dual-etch processing on SOI wafer exhibits low crosstalk and reasonable mode match. Advanced designs using this concept are expected to further improve performance.

We show how long range light matter interactions can be realized at hyperbolic metasurfaces. We concentrate on the dipole-dipole interactions which are important for energy transfer studies and in quantum entanglement between qubits at metasurfaces. We apply this to get the electromagnetically induced transparency in artificial atoms at large distances. We will present experimental evidence for large range energy transfer.

Hyperbolic metamaterials can propagate electromagnetic modes with unusually large wave numbers (extraordinary high-momentum modes). The extraordinary high-momentum modes are non-propagating or evanescent modes in common optical media. On the other hand, hyperbolic metamaterials can sustain propagating electromagnetic waves with unusually low wave numbers (extraordinary low-momentum modes). Both extraordinary high- and low-momentum modes are of special interest for a number of applications. For example, the development of these exotic modes, which are fundamental to the operation of hyperlenses, might lead to unprecedented sub-diffraction optical imaging systems. Hyperbolic metamaterials are also of special interest for radiative decay engineering.
The inevitable trade-off between optical mode confinement and the optical losses inherent to the metal component is a fundamental challenge for plasmonics and metamaterials. We have carried out numerical analysis of Rytov’s dispersion equations to model loss-compensation in metal-semiconductor hyperbolic metamaterials with extraordinary high- and extraordinary low-momentum modes. Numerical results provide a framework for the design of loss compensation schemes in hyperbolic metamaterials with extraordinary high- and extraordinary low-momentum modes.

In this project we propose and fabricate a hyperbolic metamaterials-based narrowband notch filter for the infrared regime with a center wavelength that remains fixed as the angle of incidence changes from 0 to 30 degrees for TM polarization. This novel device modifies a conventional Bragg reflector by including a middle resonance layer that opens up a narrow, highly transmissive band. To achieve angular independence, a subwavelength sized array of silver wires are inserted in a vertical orientation and permeate all 7 Si and SiO2 layers of the structure.
In this work the theoretical underpinnings are explored using Maxwell-Garnett Theory, and simulated with 3D finite element full wave electromagnetic modeling software. Simulations demonstrate that the device is capable of up to 60% transmission at a fixed center wavelength for TM polarization in the infrared.
The device is fabricated using typical microfabrication techniques. The silver nanowires are fabricated via DC electrodeposition. The angle and polarization dependent transmission, reflection and absorption of the device are experimentally measured, and scanning electron microscopy images of the structure are shown.
Though the experimental validation of this device is performed for the infrared regime, scaling the structural sizes can extend the operating regime to higher and lower wavelengths. Whether used as a stand-alone filter, or integrated into a hyperspectral array, the angle-independent response of this filter has many uses in remote sensing applications.

The functionalities of traditional optical component are mainly based on the phase accumulation through the propagation length, leading to a bulky optical component such as converging lens and waveplate. Metasurfaces composed of planar structures with artificial design have attracted a huge number of interests due to their ability on controlling the electromagnetic phase as well as amplitude at a subwavelength scale. The feasible applications based on metasurfaces include nonlinear dynamics, light beam shaping, quantum interference etc. Beside those promising characteristics, people now intend to discover the field of meta-devices, where we can attain optical properties and functionalities through changing the feature characteristics of metasurfaces in demand. They therefore pave a potential way for the development of flat optical devices and integrated optoelectronic systems and toward the far-reaching applications which are impossible previously. In this talk, four research topics for photonic applications with metasurfaces and meta-devices will be performed and discussed: high efficiency anomalous beam deflector, highly dimensional holographic imaging, versatile polarization control and metadevices with active property.

We present the design of a perfect light absorber using 3D metamaterial for operation in the mid IR region. The 3D
metamaterial is a metal half ring projecting normal to the substrate plane, which ensures wide-angle operation for the
direct magnetic coupling. The absorber is essentially a 3-layer architecture having the 3D metamaterial on top acting as
impedance matching layer to the surrounding medium, which ensures near zero reflection. A ground metal plane of
120nm thickness at the bottom layer cancels any transmission through the structure for incident electromagnetic field. A
dielectric spacer layer of thickness 100nm separates the top and bottom layers. The metal parts of the absorber are
realized using gold and the dielectric spacer layer is defined by SiN thin film deposited using PECVD. The 3D half rings
are formed from the lithographically defined 2D template by releasing residual stress in the thermally evaporated gold
thin film using ICP RIE of SiN sacrificial layer. We report an absorbance of more than 90% at a peak wavelength of
12.5μm with a FWHM of 2μm.

Optical characterization of subwavelength objects is important for biology, nanotechnology, chemistry, and materials science. Unfortunately, the information about interaction of an isolated subwavelength object with light is contained in evanescent waves that exponentially decay away from the source. Numerous techniques have been proposed to access or restore this information. In interscale mixing microscopy (IMM), a diffraction grating positioned in the near field proximity of the object is used to convert the originally-evanescent waves into propagating modes that can be detected with far-field measurements. However, far-field signal needs to be post-processed to un-couple the diffraction-limited and subwavelength responses. Several techniques, based on multiple measurements, have been previously proposed. Here, we show that with simple Fourier-transform based post processing can be used to characterize position, and optical size of the object based on a single measurement. To verify the proposed formalism, three finite diffraction gratings were fabricated. Two of these gratings contained pre-engineered “defects” that played the role of “unknown objects”, while the remaining grating was used as a reference. We demonstrate that we can identify the position and size of ~wavelength/10 object with far-field characterization. The same measurement provides a platform to analyze optical spectrum of the object. Although demonstrated in this work on example of 1D grating, IMM can be extended to 2D subwavelength imaging

We develop a theory for surface-assisted plasmon decay in metal nanostructures of arbitrary shape. We derive the rate of electron surface scattering, which facilitates plasmon decay in small nanostructures, that is determined by local field polarization relative to metal-dielectric interface and is highly sensitive to the system geometry. We show that the surface scattering can be incorporated into metal dielectric function on par with phonon and impurity scattering. We illustrate our model by providing analytical results for surface scattering rate in some common shape nanostructures. Our results can be used for calculations of hot carrier generation rates in photovoltaics and photochemistry applications.

This paper presents a study of the influence of the geometric shape on the resonance frequency of the artificial magnetic conductor (AMC) by analysis of the electric field distributions on top of the surface metallic patch inside the unit cell. It is known that various parameters such as geometry, dielectric substrate thickness, gap between patches, length and width of patch, size of unit cell, permittivity and permeability strongly affect the resonance frequency. In attempts to elucidate the miniaturization process, as reference, a metallic square patch with a unit cell of size 10 mm × 10 mm was simulated and a resonance frequency of 5.75 GHz was obtained. The device has illuminated by a plane wave with polarization in the y direction. Additionally, different geometries were performed such as triangle, hexagon, circle and cross of Jerusalem. We realized that the field distribution can be used as an physical insight to understand the AMC miniaturization process. In particular, bow-tie geometry provided considerable electrical miniaturization compared with square patch, about 1.5 GHz. The results are supported by finite element method. Our findings suggest that shift at resonant frequency may be interpreted as a variation in the net induced electric polarizability on the surface of the metallic patches.

Graphene has been demonstrated to be a promising photodetection material because of its atomic-thin nature, broadband and uniform optical absorption, etc. Photovoltaic and photothermoelectric, which are considered to be the main contributors to photo current/voltage generation in graphene, enable photodetection through driving electrons via built-in electric field and thermoelectric power, respectively. Graphene photovoltaic/photothermoelectric detectors are ideal for ultrafast photodetection applications due to the high carrier mobilities in graphene and ultrashort time the electrons need to give away heat. Despite all the advantages for graphene photovoltaic/photothermoelectric detectors, the sensitivity in such detectors is relatively low, owing to the low optical absorption in the single atomic layer. In the past, our research group has used delicately designed snowflake-like fractal metasurface to realize broadband photovoltage enhancement in the visible spectral range, on SiO2 thin film backed by Si substrates. We have also demonstrated that the enhancement from the proposed fractal metasurface is insensitive to the polarization of the incident light. In this current work, we have carried out experiments of the same fractal metasurface on transparent SiO2 substrates, and obtained higher enhancement factor on the fractal metasurface than that achieved on SiO2/Si substrates. Moreover, the device allows more than 70% of the incident light to transmit during the detection, enabling photodetection in the optical path without any significant distortion. Another possibility to make use of the large portion of transmitted light is to stack multiple such devices along the optical path to linearly scale up the sensitivity.

Optical metasurfaces are periodic or graded pattern arrays of ultra-thin plasmonic and/or dielectric nanostructures, which are intended to scatter light in manners that cannot be achieved with conventional stratified media. Recent advancements in the theoretical knowledge and fabrication methods of two-dimensional materials, such as graphene, have provided the opportunity to scale down the principles of metasurfaces to atomic dimensions and to offer graded pattern meta-sheets. We present here engineered nanostructures to tailor the beaming pattern of light scattered through such meta-sheets. We obtain designs to precisely control both the in-plane scattering of surface waves associated with the sheets and also out-of-plane scattered far-field beams into a desired direction. We then determine a set of conductivity-balancing conditions to completely confine the surface waves to the meta-sheets at highly scattering sites and demonstrate that under such criterion the propagation of guided surface waves can be described simply using Fresnel equations of plane waves. Furthermore, we cascade three sinusoidally modulated reactance surfaces to realize a broad-beam leaky-wave antenna to completely scatter the surface waves to far-field and also control the steering direction. In addition, conformal patterned 2D sheets will be explored for the first time and how to successfully design and manipulate the light wavefront. For fast and accurate designs of the flat and conformal meta-sheets, we take advantage of our superior auxiliary differential equation finite-difference time-domain (ADE-FDTD) method. Also, an integral equations (IE) model will be applied for large-area system platforms design investigation.

We study lasing in regular arrays made from aluminum nanoparticles. We show that these structures function as laser sources at visible wavelengths, even when scaled to an order of magnitude smaller areas compared to existing literature. The aluminum nanoparticles provide a robust platform for studying lasing in plasmonic systems, even when the optical losses are higher compared to silver or gold.

Nanophotonic structures that localize photons in sub-wavelength volumes are possible today thanks to modern nanofabrication and optical design techniques. Such structures enable studies of new regimes of light-matter interaction, quantum and nonlinear optics, and new applications in computing, communications, and sensing. The traditional quantum nanophotonics platform is based on InAs quantum dots inside GaAs photonic crystal cavities, but recently alternative material systems based on color centers in diamond and silicon carbide have emerged, which could potentially bring the described experiments to room temperature and facilitate scaling to large networks of resonators and emitters. Additionally, the use of inverse design nanophotonic methods that can efficiently perform physics-guided search through the full parameter space, leads to optical devices with properties superior to state of the art, including smaller footprints, better field localization, and novel functionalities.

In this paper, we propose dynamically tunable plasmon induced transparency (PIT) in a graphene-based nanoribbon waveguide structure by changing the chemical potential of graphene. It is the direct destructive interference between the propagating plasmonic edge mode in the graphene nanoribbon waveguide and the rectangular resonators gives rise to the PIT effect. Our numerical results reveal that high tunability in the PIT transparency window can be obtained by altering the chemical potential of the graphene rectangular resonators. Moreover, a novel plasmonic refractive index sensor (RIS) has been proposed and investigated numerically based on the PIT effect in the mid-IR range. Our calculated results exhibit that large wavelength sensitivity as high as 2500 nm/RIU and a high figure of merit (FOM) of 10.50 can be achieved in this ultra-compact structure (<0.05 μm2 ). This work not only paves a new way towards the realization of graphene-based integrated nanophotonic devices, but also has important applications in multi-channel-selective filters, sensors, and slow light.

In this paper we present the design of a metamaterial perfect absorber (MPA) made up of an array of dielectric microcubes grown on a metallic substrate. The fundamental principle of operation of the proposed structure is Mie Resonance occurring in high permittivity particles in combination with the negative permittivity provided by the metallic substrate. The proposed structure is simpler than all other existing metamaterial perfect absorber structures. The geometrical parameters of the structure are between 1 μm and 10 μm, hence it is not supposed to pose any challenge during fabrication. Moreover, the structure has been designed for terahertz spectrum which is the most unexplored part of the spectrum.

We present a broadband absorber with the half-cylinder geometry composed of thin film with metallic / dielectric
multilayers. The geometric and physical parameters of the proposed structure were optimized to obtain an average
absorption above 90% for the modes of electric polarization (TE) and magnetic (TM) in the visible spectrum. High
absorption is observed for incident angles of up to 40 degrees in TE mode and 80 degrees for TM mode. The effects
of structure periodicity are also investigated for both modes and the results show small changes over a range of 200
nm . In medium IR (infrared) the structure can be scalable to obtain absorption peaks from its geometry.

We demonstrate automatic design of infrared (IR) metamaterials using a genetic algorithm (GA) and experimentally
characterize their IR properties. To implement the automated design scheme of the metamaterial structures, we adopt a
digital metamaterial consisting of 7 × 7 Au nano-pixels with an area of 200 nm × 200 nm, and their placements are coded
as binary genes in the GA optimization process. The GA combined with three-dimensional (3D) finite element method
(FEM) simulation is developed and applied to automatically construct a digital metamaterial to exhibit pronounced
plasmonic resonances at the target IR frequencies. Based on the numerical results, the metamaterials are fabricated on a
Si substrate over an area of 1 mm × 1 mm by using an EB lithography, Cr/Au (2/20 nm) depositions, and liftoff process.
In the FT-IR measurement, pronounced plasmonic responses of each metamaterial are clearly observed near the targeted
frequencies, although the synthesized pixel arrangements of the metamaterials are seemingly random. The corresponding
numerical simulations reveal the important resonant behavior of each pixel and their hybridized systems. Our approach is
fully computer-aided without artificial manipulation, thus paving the way toward the novel device design for next-generation
plasmonic device applications.

We study the length propagation characteristic of plasmonic waveguides employing metamaterials by means of
numerical approach. The analyzed structures are made of metallic nanowires in a dielectric host or metal and
dielectric thin layers claddings, surrounding a dielectric core. The main parameter to be computed is the length
propagation as a function of the light excitation wavelength, waveguide core dimensions, metal filling ratio or the
effective index of the composite claddings. These structures are intended to exhibit low loss propagation guided
modes from visible light to optical communication infrared radiation.

We propose a high-Q optically tunable terahertz (THz) filter consisting of subwavelength multilayer graphene/ dielectric/metal asymmetric square split-ring resonators (SRR) within a unit cell. The obtained simulation results demonstrate that Fano resonance can be efficiently modulated under IR-radiation of different intensity value. The modulation depth of Fano resonance can achieve about 60% under the maximum considered pumping intensity (corresponding to 0.4 eV of Fermi energy) with the Q-factor of about 135. The proposed metasurface provides narrow filtering of incident light as well as sensing applications.

In this paper, a second-order nonlinear interaction, difference-frequency generation, based on three-wave mixing process
in nonlinear multilayered metmaterial has been investigated numerically. The nonlinear multilayered metmaterial is
composed of two periodically alternating metallic and dielectric layers, which their layer thicknesses are reduced into
deep-subwavelength size for creating some nonlinear optical effect enhancement mechanism. The optimal engineered
structure gives a dispersion relation having near zero permittivity at some frequencies and can be called epsilon-nearzero
point. When a pump frequency (ω1) is determined at this point,